1. Field of the Invention
The present invention relates generally to apparatus for delivering energy to optical windows of diagnostic apparatus and, more specifically, to apparatus that heat optical windows of gas analysis apparatus. In particular, the present invention relates to heaters for use on optical windows of respiratory gas analysis apparatus that provide a substantially unobstructed optical pathway through such optical windows.
2. Background of Related Art
Various types of sensors that are configured to communicate with the airway of a patient to facilitate measurement of the amounts of substances, such as gases or vapors, in the respiration of the patient are well known in the art. Similarly, techniques are well known in the art by which the amount of various substances in the patient's respiration may be measured.
Mainstream sensors are typically configured to be connected at some point along the length of a breathing circuit. Thus, as the patient breathes, respiratory gases and vapors that are flowing into or out of the breathing circuit pass directly through a mainstream sensor. Accordingly, a substantial portion of the patient's respiratory gases may be included in the measurement obtained by the sensor.
In contrast, sidestream sensors, rather than being located such that respiratory gases pass directly through the sensor, typically include small bore (i.e., inner diameter) sampling lines, or conduits, that “tap” into a breathing circuit to communicate therewith at some point along the length thereof. These small bore sampling lines draw small samples of the patient's respiratory gases from the breathing circuit. The samples then are conveyed to a sensor (e.g., a cuvette) that is located remotely from the breathing circuit. Measurements are then obtained to determine the amounts of one or more substances that are present in the sample.
By way of example, respiratory sensors such as those described above may be used in determining the amounts of molecular oxygen (O2), carbon dioxide (CO2) and anesthetic agents, e.g., nitrous oxide (N2O), that are present in the respiration of a patient. These gas analyses are useful in a variety of other medical procedures including, without limitation, monitoring of the condition of a patient in critical or intensive care, in heart stress tests of an individual as he or she exercises (typically on a treadmill), and in other tests for monitoring the physical condition of an individual, or the like.
An exemplary, conventional mainstream gas sensor comprises a so-called “airway adapter” that includes a cuvette and that is configured to be coupled into a breathing circuit. A cuvette of such an airway adapter includes a chamber with a pair of opposed, substantially axially aligned optical windows flanking a sample flow path through the airway adapter. The windows have a high transmittance for radiation in at least a portion of the electromagnetic spectrum while maintaining an airtight seal in the breathing circuit. For infrared gas analysis, the material from which the windows are formed transmits a portion of the spectrum of electromagnetic radiation that corresponds to the wavelength or wavelengths of infrared radiation.
One optical gas monitoring technique that has long been employed to facilitate the detection and monitoring of gases, such as O2, CO2, and N2O and other anesthetic agents, is infrared absorption. In infrared absorption techniques, infrared radiation of one or more wavelengths and of known intensity is directed into a stream of respiratory gases. The wavelength or wavelengths of such radiation are selected based upon the gas or gases being analyzed, each of which absorbs one or more specific wavelengths of radiation. The intensity of the radiation which passes through the stream of respiratory gases, typically referred to as “attenuated radiation,” is measured and compared with the known intensity of the radiation that was directed into the stream. This comparison of intensities provides information about the amount of radiation of each wavelength that is absorbed by each analyzed gas. In turn, information is provided concerning the amount (i.e., the concentration or fraction) of each analyzed gas that is present in the patient's respiration.
When infrared-type gas sensors are used, the respiratory gases of an individual are typically channeled, by way of a nasal canula or endotracheal tube, along a breathing circuit to a cuvette in communication therewith. If the patient is unable to breathe on his or her own, a mechanical ventilation machine may be coupled to an opposite end of the breathing circuit. Respiratory gases are channeled along a defined sample flow path that passes through the cuvette, which provides an optical pathway between a source of infrared radiation and an infrared radiation detector. In some cuvettes, the source and detector may be detachably coupled to the cuvette.
Another known optical gas monitoring method is referred to as “luminescence quenching.” Luminescence quenching has been used to measure the amount of oxygen and other gaseous or vaporized materials in respiratory samples and other gas and/or vapor mixtures. Typically, luminescence quenching requires the emission of excitation radiation from a source toward a luminescing material. The luminescing material has a luminescence chemistry, the luminescence of which may be quenched specifically by one or more types of gaseous or vaporized materials which may be measured (e.g., oxygen, an anesthetic agent, etc.). The excitation radiation to which the luminescing material is exposed causes the material to be excited and to emit electromagnetic radiation of a different wavelength than the excitation radiation. The presence of the one or more materials of interest quenches the luminescing material. Stated differently, if a luminescence quenching gas or vapor of interest is present, the amount of radiation emitted by the luminescing material will be reduced. The amount of radiation emitted by the luminescing material and the rate at which such radiation is quenched are measured by a detector and compared with the amount of radiation emitted by the luminescing material and the rate at which the luminescence of the luminescing material is quenched in the absence of the luminescence quenching gas or gases of interest. This comparison facilitates a determination of the amount of the one or more sensed, luminescence quenching gases (e.g., in the respiration of a patient) to which the luminescing material is exposed.
For instance, when luminescence quenching is used to measure the amount of oxygen in a respiratory sample, an appropriate luminescing material (i.e., at least one wavelength of the luminescence of which is quenched when exposed to oxygen) is first excited to luminescence. Upon exposure of the luminescing material to O2, the luminescence thereof is quenched. The amount of quenching is indicative of the amount of oxygen present in a gas mixture to which the luminescing material is exposed. Thus, the rate of decrease in the amount of luminescence, or quenching of luminescence of (i.e., the intensity of electromagnetic radiation emitted by) the luminescable material, corresponds to the amount of O2 (e.g., fraction or concentration of a gas mixture) to which the luminescing material has been exposed.
When luminescence quenching techniques are utilized with mainstream airway adapters, such as the techniques described above with respect to mainstream infrared gas analyzers, the luminescing material is located within the airway adapter and may be positioned adjacent to or otherwise exposed through an optical window thereof. Accordingly, the material of the window must transmit radiation of at least a wavelength that is appropriate for exciting the luminescing material and a wavelength that is quenched by an analyzed material with sufficient efficiency to provide an accurate determination of an amount of the analyzed material.
The rate at which luminescence quenching occurs when a luminescing material is exposed to an analyzed material is a strong function of the temperature of the luminescing material and, thus, of a film or other substrate upon which the luminescing material is carried. It is, therefore, desirable to either control or compensate for any variation in the temperature of the carrying substrate.
Typically, the respired gases that enter a breathing circuit are approximately at body temperature and contain a substantial amount of humidity. One problem that has arisen in the use of conventional gas sensors is the gathering of moisture on the optical windows creating condensation or fogging thereon. Such fogging or condensation creates an obstacle to the transmission of the appropriate wavelengths of the electromagnetic spectrum and, thus, may result in inaccurate measurements.
To alleviate this problem, moisture (e.g., water vapor) typically is removed from respiratory gases in sidestream sampling by way of water traps or the like, formed of moisture-absorbing materials such as NAFION®, a material which includes hydrophilic regions. In mainstream gas sensors, on the other hand, the optical windows typically are heated to prevent moisture from gathering thereon.
The optical windows of mainstream gas sensors typically are heated by way of heaters associated therewith. By way of example, a heater may comprise a block of aluminum that communicates with a thermal capacitor or other electrical heating element to receive heat therefrom. A conventional heater may be associated with an edge of the optical window or placed in proximity to the window. To avoid blocking the optical path through the window, such a heater must heat a window indirectly. As a consequence of indirectly heating the portion of an optical window through which measurements are to be obtained, it is difficult to control the temperature of that portion of the window, as well as to monitor and quickly adjust the window temperature, if necessary.
Accordingly, a heating technique that provides heat directly to an element of an optical gas sensing apparatus (e.g., an optical window) without significantly compromising the optical properties of the optical gas sensing apparatus would be advantageous.